This invention was made with Government support under Contract No. F49620-90-C-0039 awarded by the Air Force Office of Scientific Research and under Grant No. ECS-9108570 awarded by the National Science Foundation. The Government has certain rights in the invention.
The present invention relates, in general, to a self-starting mode-locked laser, and more particularly to a self-starting mechanism for a laser that enables the laser to be self-starting within milliseconds while remaining completely unperturbed by the self-starting mechanism while it is mode locked. The apparatus includes a position-modulated mirror in an external cavity in which the cavity length and alignment are noncritical.
Recently, a great deal of emphasis has been placed on the development of passive mode-locking techniques for linear cavity lasers that would lead to self-starting, stable operation. Self-mode-locking has been demonstrated to produce short pulse widths and high output power without the need for additional cavity elements or optical modulation, and has been highly desirable for that reason. The onset of self-mode-locking, which has been attributed to the optical Kerr effect inside a laser crystal, requires formation of an intensity fluctuation in the cavity. Such an intensity fluctuation can be produced, in its simplest form, by a mechanical perturbation of the cavity mirrors or by a quick translation of an intracavity prism. Other methods of initiating mode locking include transverse-mode beating, the use of an intracavity saturable absorber or glass filter, acousto-optic modulation, and the use of an external cavity with a nonlinear reflector or a moving mirror.
The use of a moving mirror to initiate the mode-locking in linear cavities has the advantages that it is uncomplicated and inexpensive, is not wavelength dependent, no intracavity dispersive elements are required, and the parameters of external cavity length and mirror modulation are not critical. However, this process has the disadvantage that it requires continuous feedback into the cavity during mode-locked operation, with the feedback being collinear with the normal output. This feedback destroys the femtosecond mode locking, resulting in picosecond mode locking, except for one particular alignment of the external cavity.